Hm. I didn't put my guess in the original question because I didn't want to bias a potential answer, but my guess was different than your answer. The BRB does say that "Open terrain is the most prevalent type of battlefield terrain". It seems that strict RAW would therefore suggest that "open" is a type of terrain, and included in anything that refers to "all terrain". Is there a FAQ to the contrary?

Hm. I didn't put my guess in the original question because I didn't want to bias a potential answer, but my guess was different than your answer. The BRB does say that "Open terrain is the most prevalent type of battlefield terrain". It seems that strict RAW would therefore suggest that "open" is a type of terrain, and included in anything that refers to "all terrain". Is there a FAQ to the contrary?

A friend raised the interesting point that considering "open terrain" as "terrain" would be absurd because it would cause Goblin Fanatics to instantly die upon entering the board, since they die upon contact with terrain.

Sort of.

The exact wording for Fanatics is "a terrain feature of any kind." The question is, does "any terrain feature" mean the same thing as "all terrain"?

I heart Warhammer. Of course there is no FAQ on Fanatics either, at least not that I could find...and it appears they have been the source of confusion for some time. For instance I found a 7th ed. thread debating whether hills (which are treated as open terrain) kill them or not. People seemed to think so.

A terrain feature is not the same as 'terrain'. A feature is something that you can clearly identify, like the edge of a forest, impassable terrain, etc. A hill is a terrain feature, it'd kill fanatics.

Another data point. It so happens I play Space Wolves in 40k, and the Rune Priest has a power that can cause skimmers and such to take Dangerous Terrain tests all over the place. The exact wording of that power is "...treat all terrain, even clear terrain, as both Difficult and Dangerous." So now we're getting somewhere. That statement is very clear, and is a time GW used the phrase "all terrain" to describe the entire game board, not words like "entire board" or "battlefield." My group has always interpreted clarifying statements such as "...even clear terrain..." as just that: clarifying statements, not additional rules. In other words, the Space Wolves power would mean the same thing even if "...even clear terrain..." were omitted; it would just be less clear. Furthermore if "all terrain" were intended to mean something other than "the entire game board", then GW would have chosen some wording other than "all terrain" for the Space Wolves power.

I believe this is pretty conclusive: the Lore of Beasts spell refers to the whole board.

By the letter of the law, you may be right. I just don't think very many people will ever play it like that. There is RAW and then there is RAI. I personally would not consider open terrain as a terrain feature. That is just reading too much into the words.

I also think you statement from 40k proves the opposite. If they had intended for it to work that way for the spell, it would have said all terrain, even open terrain. Since it didn't, I would say that they didn't intend for it to be.

So by RAI, It seems odd for me for a 10+ cast difficulty spell to require the spell target to do something easily avoidable in order to cause the spell to have any effect. 10+ seems about right for a spell that causes someone to think real hard before charging you, or fleeing from your charge, or do what amounts to a march block...then even if they ignore you, they only take wounds on a 1-2, and then can ward/regen from those. There are plenty of 10+ spells that will kill a third of a unit without giving them the option to avoid the damage by only taking normal moves. In other words, I think if you don't interpret the spell my way, it's quite underpowered for a 10+.

EDIT:
Another point I forgot to mention. The text of the spell acutally says "all terrain (except for impassible terrain)". If you look in the BRB 8th, you'll see that there are only 4 entries under "Terrain Types": Open, Dangerous, Impassible, and Mysterious. Entries like "Forests" and "Hills" are peer entries, not sub entries, of "Terrain Types". To me a reasonable interpretation of "all terrain (except for impassible terrain)" is "all entries in the 'terrain types' section, except for the 'impassible terrain' entry." Such an (I believe reasonable) interpretation would include open terrain.

SECOND EDIT (is that bad form? sorry):
Anyway the more concise way of stating the above is: the spell says "all terrain (except for impassible terrain)". If that was intended to exclude open terrain (which is listed in the same section as impassible terrain), why doesn't it say "all terrain (except impassible and open terrain)?"

THIRD EDIT (sorry I will stop if you want):
Sirkently, I actually agree with:

I personally would not consider open terrain as a terrain feature

However I also agree with The bearded one:

A terrain feature is not the same as 'terrain'. A feature is something that you can clearly identify, like the edge of a forest, impassable terrain, etc.

The spell mentions "all terrain," not "terrain features." I think "all terrain" includes open terrain; "terrain features" does not, at least not necessarily: you can be open terrain and a terrain feature at the same time (a hill is) or open terrain but NOT a terrain feature (the rest of the game board).